I use my PP as well to access lounges when flying WN. It can be a little hard some times when flying out of "Close In" airports that don't normally offer lounge service like HOU and MDW, but very useful at major airports that WN flies to like PHX and LAX.
I would love to have free access but I don't know if that would be something WN is interested in giving away. It would also seem crowded if every A-List and BS pax could access a lounge. Everytime I am in a PP lounge, there are usually only 5-6 other people in there and I find it a nice quiet space. Not sure how quiet it would be with a sudden increase of eligible customers.
Good thoughts though!